Sulfur dichloride

Sulfur dichloride is the chemical compound with the formula SCl2. This cherry-red liquid is the simplest sulfur chloride and one of the most common, and it is used as a precursor to organosulfur compounds. It is a highly corrosive and toxic substance, and it reacts on contact with water to form chlorine-containing acids.

Chlorination of sulfur
SCl2 is produced by the chlorination of either elemental sulfur or disulfur dichloride. The process occurs in a series of steps, some of which are:
 * S8 + 4 Cl2 → 4 S2Cl2; ΔH = −58.2 kJ/mol
 * S2Cl2 + Cl2 ↔ 2 SCl2; ΔH = −40.6 kJ/mol

The addition of Cl2 to S2Cl2 has been proposed to proceed via a mixed valence intermediate Cl3S\sSCl. SCl2 undergoes even further chlorination to give SCl4, but this species is unstable at near room temperature. It is likely that several S_{n}Cl2 exist where n > 2.

Disulfur dichloride, S2Cl2, is the most common impurity in SCl2. Separation of SCl2 from S2Cl2 is possible via distillation with PCl3 to form an azeotrope of 99% purity, however sulfur dichloride loses chlorine slowly at room temperature and reverts to disulfur dichloride. Pure samples may be stored in sealed glass ampules which develop a slight positive pressure of chlorine, halting the decomposition.

Use of SCl2 in chemical synthesis
SCl2 is used in organic synthesis. It adds to alkenes to give chloride-substituted thioethers. Illustrative is its addition to 1,5-cyclooctadiene to give a bicyclic thioether A well tested method for the production of the mustard gas bis(2-chloroethyl)sulfide, is the addition of ethylene to sulfur dichloride:
 * SCl2 + 2 C2H4 -> (ClC2H4)2S

SCl2 is also a precursor to several inorganic sulfur compounds. Treatment with fluoride salts gives SF4 via the decomposition of the intermediate sulfur difluoride. With H2S, SCl2 reacts to give "lower" sulfanes such as S3H2.

Reaction with ammonia affords sulfur nitrides related to S4N4. Treatment of SCl2 with primary amines gives sulfur diimides. One example is di-t-butylsulfurdiimide.

Safety considerations
SCl2 hydrolyzes with release of HCl. Old samples contain Cl2.